An important objective in elevator systems is to maximize passenger safety. The elevator car must never be allowed to fall freely, and its motion must never reach an uncontrolled acceleration of motion or an uncontrolled deceleration of motion. Therefore, the elevator apparatus comprises several different safety and stopping devices which take care of stopping the elevator car in both normal and fault situations.
The elevator control system takes care of driving the elevator from floor to floor. During normal operation, acceleration and deceleration, the elevator control system takes care of, for example, slowing down the speed of the elevator and stopping the elevator at the right floor. The control system also stops the elevator smoothly at the terminal floor. If normal stopping of the elevator by the control system does not work, then smooth stopping of the elevator at the terminal floor is taken care of by a Normal Terminal Slowdown (NTS) function.
If the Normal Terminal Slowdown (NTS) function fails to stop the elevator as it reaches the end of the shaft, then the elevator will be stopped by an Emergency Terminal Speed Limiting (ETSL) function by using the machine brake of the elevator. The machine brake is an electromechanical brake, which is generally arranged to engage the traction sheave of the elevator when necessary. If the deceleration of the elevator is not sufficient, then ETSL can additionally use the brake of the elevator car or the wedge brake, i.e. safety gear, to stop the elevator.
FIG. 1 illustrates the operation of the safety devices of an existing elevator system. The graph 10 represents the movement of the elevator as a function of distance and velocity.
The safety device used may be a mechanical overspeed governor (OSG). The overspeed governor monitors the velocity of the elevator car in the elevator shaft and, if the velocity of the elevator car exceeds a given preset limit value (e.g. 6 m/s), then the overspeed governor will break the safety circuit of the elevator, causing the machine brake to engage (area 12). The elevator has a safety circuit that will break when one of the switches connected to it opens. If the overspeed still goes on increasing, then the overspeed governor will operate the safety gear (area 16) provided in conjunction with the elevator car, the safety gear wedge engaging the elevator guide rails and preventing movement of the elevator car. In other words, if the ropes or rope suspensions fail and the elevator car starts falling freely, then the safety gear will get wedged and seize.
Placed near the end of the elevator shaft is a final limit switch. The position of the final limit switch is indicated by x1 in FIG. 1. If the elevator has not stopped before reaching the final limit switch, then the elevator safety circuit is broken again and the brake of the elevator is activated. The final limit switch uses the machine brake (range 12) to stop the elevator car if the elevator advances e.g. 100 mm beyond the final position.
If the elevator moves on a few centimeters past the final limit switch, then the car (or correspondingly counterweight) will hit a buffer 13, which will finally stop the elevator by a springing action. Even after the buffer there must be an empty space 14, after which the elevator would meet the concrete end structure 15 of the shaft.
Even if the normal control system should fail, full-sized buffers have a sufficient stroke length such that it is in principle safe to run at full speed onto the buffers without the acceleration inside the car exceeding the allowed limit before the elevator car stops. Typically 1 g is an acceleration/deceleration level defined in safety regulations as a value that a human being can withstand.
FIG. 2 illustrates the operation of the safety system of an elevator when the elevator system uses a so-called reduced-stroke buffer 23. After the buffer 23 there is an empty space 24, which is adjacent to the concrete end structure 25 of the shaft. In this case, the stopping of the car is implemented utilizing an electric safety circuit. Mounted at a certain distance from the end of the shaft is a switch having a speed limit of e.g. 90% of the nominal speed (switch 2 and switch 3). Another switch, having a speed limit of e.g. 60% of the nominal speed (switch 1 and switch 4), is mounted closer to the shaft end. If the speed is over 90% of the nominal speed at the switch (switches 2 and 3), then the safety circuit will be broken again and the machine brake (area 22) will stop the elevator car. If the speed is over 60% of the nominal speed at the switch (switches 1 and 4), then the safety circuit will be broken and the machine brake will stop the elevator car (area 22). If the overspeed still increases from that level, then the elevator's safety system will use the safety gear provided in conjunction with the elevator car to stop the car (area 26).
The authorities of different countries have different regulations regarding the safety of elevators. The basic principle is that the elevator should have a safety system that is capable of stopping the elevator in a fault situation. For example, according to elevator directive 95/16/EC of the European Union, an elevator should be provided with an overspeed governor and a speed monitoring system. The elevator must not reach an uncontrolled acceleration of motion or an uncontrolled deceleration of motion. In addition, in the elevator shaft, between the elevator car and the end of the elevator shaft there must remain buffers and a sufficient safety space.
When elevators are renewed in old buildings, problems often arise because the safety regulations have changed over the years and there are no sufficiently large spaces in the elevator shaft above and below the elevator car as required by the current safety regulations. Extending the shaft upwards or downwards is in most cases impossible in respect of construction engineering or at least so expensive and difficult that it is out of the question.